The PET Image Analysis Core will provide a shared resource for implementation and standardization of image analysis software compatible with the Siemens ECAT EXACT HR scanner. The Image Analysis Core will test and implement software for alignment of images with subjects including within study and across study alignment of H2 15O,C15O and O15O images and registration of PET with CT images. A correction for partial volume effect from intracerebral hematomas and cerebrospinal fluid bases on the CT scan also will tested and implemented. The effect of different reconstruction filters and different region of interest sizes on regional physiological measurements will be investigated to determine the best combination to maximize both accuracy and spatial resolution. Software to permit analysis of within subject changes based on generation of histograms of defined ranges of values of physiological measurements will be tested and implemented. Standardized image processing will be applied to a control group of patients with carotid occlusion and normal Ct scans who were asymptomatic at the time of PET to provide data on minimum regional values rCBF and rCMRO2, OEF and CvO2 that are still compatible with brain function and viability.